Capacitive sensors have become increasingly common and accepted for use in human interfaces and for machine control. In the field of home appliances it is now quite common to find capacitive touch controls operable through glass or plastic panels. These sensors are increasingly typified by my U.S. Pat. No. 6,452,514, the disclosure of which is herein incorporated by reference, which describes a matrix sensor approach employing charge-transfer principles.
Due to increasing market demand for capacitive touch controls, there is an increased need for lower cost-per-function as well as greater flexibility in usage and configuration. Similarly, there is a significant demand for capacitive material displacement sensors (e.g. fluid level sensors, mechanical movement sensors, pressure sensors etc.) at lower price points, which cannot be easily met with current generations of non-mechanical transducers.
In many applications there is a need for a human interface having many keys or sensing positions, akin to the flexibility afforded by 2-D touch screens or touch pads, as typified by U.S. Pat. No. 4,476,463 (Ng) or U.S. Pat. No. 5,305,017 (Gerpheide). For example, in a computer monitor, it is desirable to have controls on the screen bezel to allow adjustment of brightness and contrast. Ideally a continuously adjustable control (e.g. a potentiometer) is used to control these parameters. Due to price pressures and aesthetic requirements, these have been usually eliminated in favor of a few bezel-mounted menu selection buttons which are much harder for the user to understand.
In the fields of electronic and medical test instruments, LCD displays are often used in conjunction with rows of bezel buttons to provide software-driven menu functions. Many such applications are incompatible with the expense, reduced contrast, and fragility of touch screens. Moreover, the performance of some of these suffer from deep or limited menu options and visual parallax. An example of this type of menu control is to be found in almost any current cash dispenser such as the LCD-5305 model made by the NCR Corporation of Dayton, Ohio. Manufacturers would use higher resolution controls on or closer to the edge of the screen if economic considerations could be met. Similar markets exist for domestic appliances, educational games, info/internet kiosks, and the like.
In the field of HVAC, the state of the art in wall-mount thermostatic controls is currently exemplified by the Model CT8602 made by the Honeywell Corporation of Morristown N.J. This model is a menu-drive system with a small LCD screen. Advanced features in these devices are accessed via deep levels of menus which are often non-intuitive when compared with simple dial or slider based controls.
Electromechanical human interface controls (such as pushbuttons, membrane switches, and potentiometers) have the noted defects of being unreliable and subject to water ingress, as well as being only marginally compatible with LCD-based menu systems. Classic user controls, such as dials and resistive potentiometers, require panel openings which allow dirt and moisture to enter into the product. They also do not present a ‘clean’ appearance, are considered increasingly quaint, and seriously limit the flexibility of industrial designers. U.S. Pat. No. 5,920,131 (Platt) describes one solution to this problem in the form of a rotary knob which is magnetically held to a seamless panel surface and which magnetically interacts with position sensing detectors below the panel surface. This solution still requires a knob, is expensive to manufacture, and only works well as a rotary device, although many applications require linear sensing.
There exists a substantial demand for new human interface technologies which can, at the right price, overcome the technical deficits of electromechanical controls on the one hand, and the cost of touch screens or other exotica on the other. It is the intent of the invention to permit a new class of touch sensor which addresses these issues in unison, while providing also the basis for a whole new class of position sensors across a number of industries and applications.
In the field of mechanical displacement sensing LVDTs, exemplified by the Schaevitz (Slough, UK) MP series, exist to provide precision positioning information for feedback in process controls. Other smaller devices such as the Schaevitz XS-B series are incorporated into machines and instruments. Such devices are usually high-cost solutions, albeit very accurate, and rely on magnetic field balance measurements made with expensive signal conditioners. These devices exist to provide highly reliable non-contact sensing and can operate in harsh environments with great precision. They solve the wiper-reliability problem of resistive potentiometric methods by eliminating the use of a physical contact. Similarly, there are capacitive position sensors, as exemplified by the RDP Electrosense Inc. (Pottstown, Pa. U.S.A.) RCDT capacitive transducer, which also requires a special, expensive signal conditioner to operate. An example of such technology is described more fully in U.S. Pat. No. 5,461,319 (Peters) which describes a bridge based circuit. Capacitive based devices can accommodate both linear and rotational position sensing. For example U.S. Pat. No. 5,079,500 (Oswald) describes a linear or rotary ‘potentiometer’ having a capacitive wiper, which makes for a highly reliable method of position sensing as it does not use a galvanic wiper. Adaptations are available to measure pressure and by inference, fluid level. The above referenced technologies however suffer the problem of being very complex and expensive to manufacture, limiting their use to high-end or industrial equipment.
LVDT and RCDT type transducers work very well, but leave untapped a very large market for low cost devices which can be used commercially in automotive and appliance applications. It is a further intent of the invention to permit the creation of a new class of position sensor, based on capacitance, that does not also require expensive signal conditioning or the need for expensive wound coils or magnets and that is adaptable to either linear or rotary position sensing.
Capacitive fluid sensors which measure capacitance change caused directly by the fluid exist; one example is contained in my U.S. Pat. No. 6,457,355. Other examples abound, such as U.S. Pat. No. 6,178,818.
Existing capacitive fluid sensors are not very popular due to their cost or inability to compensate automatically for changes in dielectric properties. Few are capable of accurately sensing fluid level from outside a plastic or glass vessel. It is a further intent of the invention to provide for a new type of capacitance based fluid sensors that can economically sense the level of contents of a container from inside or outside, without regard to the dielectric properties of the material or fluid being sensed.
In my U.S. Pat. No. 5,730,165, I teach a capacitive field sensor employing a single coupling plate to detect change in capacitance to ground. This apparatus comprises a circuit employing repetitive charge-then-transfer or charge-plus-transfer cycles and preferably uses CMOS switching elements that differ from common integrated CMOS push-pull driver circuitry in that the CMOS elements disclosed in U.S. Pat. No. 5,730,165 have floating terminals. In my subsequent U.S. Pat. No. 6,288,707, I teach the use of this charge transfer technology for sensing position in one and two dimensions. The disclosure of U.S. Pat. Nos. 5,730,165 and 6,288,707 are herein incorporated by reference.
In my U.S. Pat. No. 6,466,036 “Charge Transfer Capacitive Measurement Circuit”, the disclosure of which is herein incorporated by reference, I teach another capacitive field sensor employing a single coupling plate to detect change in capacitance to ground. This apparatus comprises a circuit employing repetitive charge-then-transfer or charge-plus-transfer cycles and, significantly, uses common integrated CMOS push-pull driver circuitry in which one terminal of every switch is connected either to a reference voltage or to a circuit ground. The disclosure of U.S. Pat. No. 6,466,036 is herein incorporated by reference.